Abstract Retained hemothorax is a complication of bleeding into the pleural space that is associated with chest trauma, malignancy, anticoagulation or hemostatic disorders, affects thousands of US patients annually, promotes respiratory compromise, is difficult to manage and is now mainly treated surgically. Many patients are poor surgical candidates and decortication is a major chest surgery that entails significant morbidities. The literature suggests that intrapleural fibrinolytic therapy (IPFT) can be salutary, but there are no agents approved for RH, nor are there established dosing schedules. Pharmacologic interventions for RH are desirable but preclinical testing has been hampered by the lack of suitable preclinical models to test new strategic approaches. This project addresses that gap through the development and validation of a rabbit RH model amenable to such testing. The few prior models reported in the literature are not suitable for this testing and have hemostatic and fibrinolytic systems that differ importantly from humans. We have extensive experience in the successful creation and use of rabbit models that like RH are characterized by pleural organization, as in tetracycline-induced pleural injury or empyema. We have used the models to advance new therapeutics for empyema, partnered with NHLBI SMARTT and have brought one such agent; single chain urokinase plasminogen activator (scuPA) to phase I clinical trial testing. We also founded a company; Lung Therapeutics, Inc. (LTI) that has attracted sufficient funding to independently bring scuPA to international phase II testing, which is scheduled to begin in the first quarter of 2020. Our preliminary data demonstrate that we can generate the model and use validated performance measures to accomplish the work that we envision will ultimately be used to guide dosing of scuPA or the use of other promising agents for near-term clinical trial testing. This project is of interest to LTI and fits the mission of the NHLBI. Our sole aim is to generate the rabbit RH model which will be characterized and validated using the well-vetted performance measures for each subaim. In the subaims, we will establish the optimal conditions to generate the model based on our experience and preliminary findings. We will use state- of-the-art small animal chest CT and ultrasound imaging, pulmonary function testing and morphometric analyses of tissue injury as performance measures to validate development of the model and test the effects of age and gender on RH outcomes. The model will also advance understanding of the pathogenesis of pleural organization associated with RH, which likely differs from other forms of pleural injury. Our team is uniquely positioned to accomplish our objective and generate the rabbit model. By again partnering with NHLBI through Catalyze, we will in future use the model to generate preclinical data to enable clinical trial testing of scuPA of RH, which may involve the support o...